The present invention relates to a method for making security systems more tamper resistant and a security system configured in accordance with the teachings of the method.
U.S. Pat. No. 4,310,835 (Sefton) teaches one skilled in the art to position an additional resistive device at the furthest extremity of each detection circuit. The purpose of this additional resistive device is to ensure that the detection circuit always contains some resistance which a control processor can monitor in order to detect tampering.
Security systems utilizing the teachings of Sefton remain vulnerable to tampering by criminals who are sufficiently sophisticated to test for in line resistance and jumper the wiring of the detection circuit with an appropriate resistance. The jumpering of the wires with a resistance equal to that of the additional resistive device fools the control processor into thinking that the detection circuit is still operational.
What is required is a method for making security systems more tamper resistant and a security system configured in accordance with the teachings of the method.
According to one aspect of the present invention there is provided a method for making a security system more tamper resistant. The method involves the step of monitoring an electric reference characteristic in a detection circuit of a security system and generating random periodic changes to the electrical reference characteristic in order to detect any tampering with the electric reference characteristic in the detection circuit.
In order to circumvent a security system, an output signal value must be ascertained in order to communicate that value back to the monitoring unit. With the method, as described above, the electric reference characteristic is randomly changed on a periodic basis. With the output value changing in a random manner, even security personnel responsible for installing and maintaining the security system are unable to predict the sequence of changing output values of the electric reference characteristic. The electric reference characteristic most commonly used in the industry is resistance which is measured as an output voltage. There are, however, other electric reference characteristics which could be monitored such as capacitance, impedance and inductance.
There are different ways in which the electric reference characteristics being monitored can be changed. Beneficial results have been obtained by providing a plurality of alternative line connections, each of which has a different electrical reference characteristic. Random periodic changes to the electrical reference characteristic are generated by connecting the detection circuit to a selected one of the plurality of alternative line connections to alter the electrical reference characteristic. The plurality of alternative line connections can be, but do not necessarily have to be positioned internally within a monitoring unit. In order to increase the number of alternative line connections, it may be desirable to position alternative line connections both internally within the monitoring unit and external to the monitoring unit.
According to another aspect of the present invention there is provided a security system which includes a control processor and at least one detection circuit monitored by the control processor. The detection circuit has at least one detector. Means is provided for generating random periodic changes to a monitored electrical reference characteristic in order to detect any tampering with the electric reference characteristic in the detection circuit. This feature can be incorporated into the existing control processor or included through the addition of a separate monitoring unit which checks for tampering.